This invention relates to a hydraulic drive system capable of operating a plurality of hydraulic drive members found in heavy equipment, such as, excavators, backhoes, and the like.
There are many different types of heavy construction equipment that use hydraulic drive systems for transmitting energy to the operating parts of the machinery in question. Machines of the type under discussion include a backhoe or shovel-type excavator, which basically has six main functions and several auxiliary functions. By way of example, this application will deal with the six main hydraulically operated functions of an excavator, which includes swing, crowd, tool, hoist, and the two motors for driving the separate tracks of a track-mounted excavator. A novel hydraulic system using one prime mover to drive a plurality of pumps is disclosed in U.S. Pat. No. 3,910,044 to Symmank for this excavator example, and reference may be made thereto for a detailed description. It is to be understood that the novel arrangement of a plurality of prime movers and pumps in a hydraulic system disclosed herein can be utilized wherever it is desired to have a simple summating system to accomplish any combination of pressures and compensate instantly for the power requirements of multiple pump units driven by a plurality of prime movers.
In order to facilitate an understanding of the application of the novel hydraulic system, a brief description of the operation of an excavator is believed to be in order.
Generally speaking, the swing movement of an excavator requires the use of a positive displacement motor or hydraulic motor for rotating the upper structure of the excavator about a turntable and about a vertical axis. The crowd movement involves the use of one or more hydraulic cylinders that are interconnected between the boom and the dipper stick of the excavator for providing pivotal movement therebetween. The tool or bucket function also involves the use of a hydraulic drive cylinder for pivoting the bucket about a horizontal pivot point at the outer end of the dipper stick. The hoist involves the use of one or more hydraulic drive cylinders for lifting the hoist and dipper stick and pivoting them about a horizontal axis on the body of the excavator. As previously mentioned, a variety of other functions may also be performed by the use of hydraulics, but in this application, we will deal with the major ones referred to above.
As can be appreciated from the above, the operator has usually four, and often more, functions to control simultaneously. In conventional hydraulic systems for an excavator, in order to accomplish the constant speed or desired pressure, which may be required for each function, there have been utilized separate hydraulic pumps, each of which is operated by a common prime mover. In such an arrangement, obviously, the combined demand for all the pumps cannot exceed the available engine power and each of the individual pumps is provided with the capability of putting out the required level of energy to operate the particular function it is designed to control. With such a system, it is clear that the total available engine power can only be used when all of the pump and machine functions are being operated simultaneously.
Another arrangement which has been used is to employ a number of pumps, each of which is designed to handle a plurality of individual hydraulic functions. Such a system is economical in that it is designed to transmit a portion of the available engine power to a series of functions and respond in a completely modulating manner with respect to a power demand of any one, or combination of functions. In such a circuit, maximum utilization of the hydraulic energy supplied to said series of functions is accomplished. Specifically, power summation is obtained when varying pressures or flows are required of any function with respect to other functions. As applied to excavators, there have been used a pair of systems that are essentially two-pump, fixed-displacement, split-horsepower systems, which function to economically summate power requirements within each of the two split power units.
Other attempts at obtaining maximum utilization of a common prime mover include very expensive, highly complicated variable-displacement pumps, but these systems are subject to the aforementioned disadvantages.
In the U.S. Pat., No. 3,910,044, to Symmank, there is provided a system which in an illustrated preferred embodiment includes a plurality of fixed displacement pumps that are operated by a common prime mover. As in the previous systems, each of these pumps is set up to economically summate the power requirements of the hydraulic functions that each pump is set up to provide. However, while these systems operate in this fashion, they are further combined so that the total engine power is available to only one of the pumps or the engine power is split among the pumps according to the power demand of all of the hydraulic functions, which thus results in the total engine power being available on demand for only one or any combination of the hydraulically operated functions of the excavator. For example, if one of the systems was being used to control the hoist cylinder, tool cylinder, and one of the track motors and the "on demand" requirements of that system at a given instant required the total engine power available for both pumps and the other operations of the excavator were not demanding of any of the power available by the prime mover, then the pump controlling the hoist, tool and track would have available to it the total engine power. Similarly, if one pump was called upon, for example, to supply pressure requiring two thirds of the total engine power and the other pump one third, the novel hydraulic summating system forming the essence of the present invention would provide this mode of operation.
Thus, maximum engine power is available to all of the hydraulic functions and can be utilized at all times, even though several of the functions may not be making demands on the engine at any given time. In effect, what is happening is that available hydraulic energy is being diverted into the system where it is needed.
The use of a summating system with a plurality of pumps driven by a common prime mover provides for the total prime mover power to be available to each of the fixed volume pumps in the system while permitting the pumps to divide up the power in whichever way the demands are present. If the hydraulic systems become larger and if the excavating machines actuated by them require more power, a larger and more powerful prime mover is required. As the prime mover size increases, the initial cost and service cost increases. It is generally known that relatively small electric motors can be used as prime movers and that their cost is much less than relatively large electric motor prime movers. In fact, owing to the larger quantity of small motors produced and the economies of scale associated with such production, the cost of a number of total small motors equivalent in total power to a large motor is less than the cost of the large motor. For example, it is not uncommon today to find that the cost of two 225 hp motors is only a little more than one-half of the cost of one 450 hp motor.
From the foregoing, it is seen that it is desirable to use a number of smaller motors in place of one larger motor to furnish the increased power requirements of a larger hydraulically operated machine.
If a number of smaller prime movers are to be used in place of one large prime mover, it is additionally desirable to be able to arrange the prime movers and driven pumps in combination with a summating system so that maximum engine power of each prime mover is available to its connected pumps in the hydraulic systems while permitting the pumps to divide up the power in whichever way the demands are present.